1. Field of the Invention
The present invention relates to distance measuring apparatuses, and more particularly, to a distance measuring apparatus such as a distance measuring sensor using a semiconductor position sensitive photodetector (hereinafter referred to as a PSD).
2. Description of the Background Art
The PSD as a light receiving element is a sensor to which a photodiode (PD) is applied for detecting a light spot position.
The operational principle of the PSD will now be described with reference to FIG. 11. A PSD 5 is formed of a silicon chip constituted of three layers of a p.sup.- layer 101 at the surface, an n.sup.+ layer 103 at the rear face, and an i layer 102 therebetween. When a light spot .phi. is irradiated onto the surface of the PSD 5, generated electrical charges (carriers) are divided in a resistance layer (p.sup.- layer) in reverse proportion to distances between the light incident position and electrodes A, B for output, and taken out from respective electrodes A, B as currents I.sub.1, I.sub.2.
As shown in FIG. 11 (A), assuming that the distance from the middle point of the electrodes A and B to a light incident position point P is x, a resistance value from the incident position point P to the electrode A is R.sub.01, a resistance value from the incident position point P to the electrode B is R.sub.02, the distance between the electrodes A and B is L, a resistance value between the electrodes A and B is R.sub.T, and currents taken out from the electrodes A, B are I.sub.1 and I.sub.2, respectively, the currents I.sub.1, I.sub.2 are given as follows: ##EQU1##
A photoelectric current I.sub.0 is given as follows: EQU I.sub.0 =I.sub.1 +I.sub.2 ( 3)
Since distribution of resistivity R.sub.i of the surface resistance layer (p.sup.- layer) is uniform as shown in FIG. 11 (B), the resistances R.sub.01, R.sub.02 are proportional to the distances from the incident position point P to the electrodes A, B, and given as follows: ##EQU2##
Substitution of the above equations for equations (1) and (2) gives the following equations of the currents I.sub.1, taken out from the electrodes A, B: ##EQU3##
Taking the ratio of the sum and the difference of the currents I.sub.1, I.sub.2, the following equation is given: ##EQU4##
In this way, when the PSD is used as a light receiving element, direct positional information x can be obtained as an output.
As described above, in the PSD, the balance of pulled-out signal currents I.sub.1 and I.sub.2 changes depending on positions of incident light spots. FIG. 12 shows the detection principle of the distance measuring sensor using the PSD.
As shown in FIG. 12, light emitted from an infrared light emitting diode (LED) mounted on a print wiring board (PWB) 22 passes through a lens 2 mounted on the front face of a housing case 21 to be reflected by an object 3 (a man as an example) to be detected to be incident on the PSD 5 mounted on the PWB 22 through a lens 4. The position (the light spot position) where the reflected light M is incident on the PSD 5 changes depending on a distance D between the man 3 and the sensor. If the object 3 to be detected becomes remote (if the D becomes larger), light is reflected by the object 3 as shown by a dashed line M1 of FIG. 12, causing the spot position of the light incident on the PSD 5 also to change. When the spot position of light incident on the PSD 5 changes, the balance of the signal currents I.sub.1 and I.sub.2 taken out from both ends of the PSD 5 changes accordingly.
By detecting the balance of the signal currents I.sub.1 and I.sub.2 in a signal processing circuit (not shown) formed on the PWB 22, it is possible to detect the distance between the object 3 to be detected and the sensor, making it possible to use the PSD 5 as a distance measuring sensor.
FIG. 13 shows a function block of a conventional distance measuring sensor using the PSD. Referring to FIG. 13, a signal processing circuit 8 processes a signal current obtained from the PSD 5, and a LED driving circuit portion drives a LED 1.
FIG. 14 shows an example of the signal processing circuit 8 for processing the signal currents I.sub.1 and I.sub.2 of the PSD 5. Referring to FIG. 14, R1 to R7 show resistances, and P.sub.1 to P.sub.5 show amplifiers. The signal currents I.sub.1 and I.sub.2 of the PSD 5 are converted to voltages V.sub.01 and V.sub.02, respectively, in a current/voltage converting circuit 11. In other words, EQU V.sub.01 =R.sub.1 .times.I.sub.1, and EQU V.sub.02 =R.sub.1 .times.I.sub.2
Subtraction of V.sub.01 from V.sub.02 is carried out in a subtracting circuit 12 to obtain an output voltage V.sub.OA corresponding to I.sub.2 -I.sub.1. V.sub.OA is given as follows: ##EQU5##
Addition of V.sub.01 and V.sub.02 is carried out in an adding circuit 13. In FIG. 14, V.sub.03 is given as follows: ##EQU6##
An output V.sub.OB corresponding to (I.sub.1 +I.sub.2) can be obtained. V.sub.OB is given as follows: ##EQU7##
By processing V.sub.OA and V.sub.OB in a microcomputer and the like, V.sub.OA /V.sub.OB is calculated. V.sub.OA /V.sub.OB is given as follows: ##EQU8##
Therefore, as described above, since (I.sub.2 -I.sub.1)/(I.sub.1 +I.sub.2) corresponds to the position of the light incident on the PSD 5, the spot position x of the light incident on the PSD can be found by the value of V.sub.OA /V.sub.OB, that is, (I.sub.2 -I.sub.1)/(I.sub.1 +I.sub.2).
When the spot position of the light incident on the PSD 5 is found, the distance between the sensor and the object 3 to be detected can be found as shown in FIG. 12.
In this way, by processing the signal currents I.sub.1 and I.sub.2 of the PSD 5 in the signal processing circuit 8, it is possible to detect the distance between the sensor and the object 3 to be detected.
FIG. 15 shows an example of another signal processing circuit 8 of the PSD 5. The circuit of FIG. 15 is constituted of a logarithm converting circuit portion 15, and a differentially amplifying circuit portion 16. Outputs V.sub.01 and V.sub.02 of log diodes 17, 18 included in the logarithm converting circuit portion 15 are given as follows, where k is Boltzmann's constant, T is absolute temperature (.degree.K.), and q is an amount of electrical charge of electrons: ##EQU9##
An output V.sub.0 provided from the differentially amplifying circuit portion 16 is given as follows: ##EQU10##
From the circuit, an output corresponding to log (I.sub.1 /I.sub.2) can be obtained. Since I.sub.1 /I.sub.2 corresponds to the spot position x of the light incident on the PSD, the spot position of the light incident on the PSD can be found by the log (I.sub.1 /I.sub.2). When the spot position of the light incident on the PSD 5 can be found, as described before, it is possible to detect the distance between the sensor and the object 3 to be detected.
In a conventional distance measuring sensor, if there is a noise source of, for example, an inverter lamp and the like in the vicinity of the sensor, noise is generated in the signal currents I.sub.1, I.sub.2 of the PSD 5 and in the signal processing circuit 8 of the PSD 5, whereby accurate detection of the balance of the signal currents I.sub.1 and I.sub.2 of the PSD 5 is hampered, which, in turn, prevents accurate detection of the distance between the object 3 to be detected and the distance measuring sensor.
When the housing case 21 shown in FIG. 12 is fabricated of, for example, polycarbonate resin (a coefficient of linear expansion of which is 70 ppm/.degree.C.), and the print wiring board 22 of the signal processing circuit 8 and the like is fabricated of glass epoxy copper-clad laminate (a coefficient of linear expansion of which is 13 ppm/.degree.C.), if environmental temperature of the sensor changes, the relative positional relation between the light receiving lens 4 fixed to the housing case 21 and the PSD 5 fixed to the print wiring board 22 changes because of the difference of the coefficients of linear expansion. If the positional relation, the positional relation in the lateral direction (the up-to-down direction in the figure) in particular, between the light receiving lens 4 and the PSD 5 changes, even if the distance D between the object 3 to be detected and the distance measuring sensor does not change, the spot position of the light incident on the PSD 5 changes. As a result, the balance of the signal currents I.sub.1 and I.sub.2 of the PSD 5 changes, causing inaccurate detection of the distance measuring sensor.
FIG. 16 is a temperature characteristic diagram of the conventional distance measuring sensor. Distance characteristics between the sensor and the object 3 to be detected are measured at each of temperatures of -10.degree. C., 7.degree. C., 25.degree. C., 39.degree. C. and 60.degree. C. selected as a predetermined temperature. From this diagram, it can be seen that a substantial output error is caused depending on environmental temperature.
In the above-described conventional technique, as shown in FIG. 13, since driving of the LED 1 is controlled only by a driving circuit portion 9, control of the amount of emitted light remained constant independently of the distance to the object to be measured. Therefore, for example, as shown in FIG. 12, when the PSD 5 is used, if it is intended to measure an object located at a long distance, it is necessary to increase the amount of received light.
However, if the amount of received light is increased, the generated photoelectric current exceeds an allowable photoelectric current of the PSD 5 in case of measurement of an object located at a short distance, resulting in a limit to the range of measurable distance.
Referring to FIG. 17, (A) shows an equivalent circuit of the PSD 5, while (B) shows an equivalent circuit in a reverse bias V.sub.R. An allowable photoelectric current I in the circuit is given as follows: ##EQU11##
where V.sub.DF is a forward voltage of a diode.
Since the apparatus of the conventional technique includes only one set of the PSD 5 and the signal processing circuit 8, for example when the PSD 5 of a chip size shown by a solid line of FIG. 18 is used, light reflected by the object 3 to be detected passes through the lens 4 to be incident on the PSD 5 if the object 3 is between points A and B. In other words, the distance range between B and A is the range of measurable distance, and distance characteristics of the output of the PSD 5 become as shown by a solid line of FIG. 19.
In this conventional configuration, when it is desired to broaden the measurable range to from point C to A, the PSD 5 is made large as shown by a dashed line of FIG. 18. As a result, the distance between C and A becomes the distance measurable range, thereby making it possible to obtain distance characteristics of the output as shown by a dashed line of FIG. 19. However, as shown in FIG. 19, when the distance between the object 3 to be detected and the sensor is long, that is, the object 3 is in the vicinity of the point A, the gradient (an amount of the output change to the distance change) of the dashed line becomes smaller as compared to that (the amount of the output change to the distance change) of the solid line. In other words, a distance measuring error becomes larger in the characteristics shown by the dashed line than in those shown by the solid line, leading to poor accuracy of measurement.
As described above, the distance measurable range and the accuracy of measurement are characteristics conflicting with each other. In the conventional technique, there was a problem that the broader distance measurable range brought the poor accuracy of measurement and that the better accuracy of measurement brought the narrower distance measurable range.
The output current of the PSD of the distance measuring sensor is very slight. For example, when the distance D between the reflecting object 3 and the PSD 5 shown in FIG. 12 is 40 cm and 80 cm, the output currents of the PSD 5 are about 1.times.10.sup.-8 A and about 3.times.10.sup.-9 A, respectively.
Therefore, there was a problem that, if there is a noise source (for example, of an inverter fluorescent lamp and the like) in the vicinity of the distance measuring sensor, the distance measuring sensor is affected by the noise, making it impossible to obtain accurate distance information (distance measuring output).
FIG. 20 is a cross-sectional view showing the state where noise is incident on an IC chip containing a LED driving circuit, a PSD signal processing circuit and the like of the conventional distance measuring sensor.
As shown in FIG. 20, an IC 7 containing an IC chip 6 is mounted on the PWB 22. When there is a noise source in the vicinity of the IC chip 6, noises 29a, 29b as shown by arrows in the figure enter. As a result, the output current is affected by the noises, leading to inaccurate distance measurement.